This invention relates generally to inverter-machine drive systems, and more specifically, to a control apparatus for an inverter-machine drive system.
In various industrial applications, it is desirable to vary the speed and torque of a polyphase alternating current machine in response to operator commands. Since alternating current machine speed and torque are dependent on the frequency and amplitude, respectively, of machine stator current, control of machine speeds and torque can be achieved by conditioning, that is to say, varying the frequency and amplitude, respectively, of machine stator current.
Commonly, conditioning of the current supplied to the alternating current machine is accomplished by energizing the machine from an inverter coupled to a source of variable potential, usually a phase controlled rectifier. Technically, the inverter is configured of a plurality of pairs of serially coupled switching devices, corresponding in number to the number of machine phases, with each pair of switching devices coupled across the phase controlled rectifier output and coupled to the junction between switching devices to a respective machine phase. The inverter switching devices, each comprised of either a high current transistor or a thyristor, are rendered conductive in a predetermined sequence to supply the machine with alternating current of varying frequency and amplitude.
Various methods presently exist for controlling inverter switching device conduction to condition inverter output voltage. The most common method and perhaps the most effective, is that of pulse width modulation whereby each of the switching devices of each pair are rendered conductive more than twice during each inverter output voltage cycle, with the switching devices of each pair being rendered conductive in sequence. Rendering of the inverter switching devices conductive in the manner described above results in each phase component of inverter output voltage being comprised of a series of positive and negative pulses. By varying the frequency of switching device conduction, the number of pulses, and hence the frequency of inverter output current, can be varied accordingly. The pulse width or pulse duration, and hence the amplitude of inverter output voltage, is controlled by varying the inverter switching device conduction duration.
Pulse width modulated inverter operation is commonly accomplished by the triangle interception technique. Each of n sine waves, where n is the number of inverter switching device pairs with the sine wave varying jointly in frequency and amplitude in accordance with operator commands and each sine wave being displaced from another by 360.degree./n, is compared against a triangle waveform of fixed amplitude and frequency, the triangle waveform frequency being typically a minimum of 6 or 9 times greater than the sine wave frequency. During the interval when each sine wave is of an amplitude greater than the triangle waveform amplitude, one of the switching devices of each respective pair of inverter switching devices is rendered conductive. Conversely, during the interval when each sine wave is of an amplitude less than the triangle waveform amplitude, the other switching device of each respective pair of switching devices is rendered conductive. Control of inverter switching device conduction in this manner assures that the fundamental frequency of each phase component of inverter output voltage is approximately the same frequency as the sine wave frequency. By varying the sine wave amplitude, the inverter modulation index, that is, the ratio of the sine wave amplitude and the triangle wave amplitude, and hence the inverter output voltage amplitude, can be varied accordingly. If the chopping ratio, that is, the frequency ratio of the triangle waveform to the sine waveform is maintained greater than 6:1, harmonics in the inverter output voltage waveform will be of a relatively high order so that inverter voltage waveform will be relatively free of harmonic distortion.
While pulse width modulated inverter operation is desirable at low inverter output voltage levels and low machine speeds, to obtain maximum inverter output voltage and maximum machine speed, inverter square wave operation is required wherein each inverter switching device is rendered conductive only once during each cycle for a duration of 360.degree./n. For a three phase inverter, operating in square wave mode, the inverter output voltage waveform appears as a six step square wave whose appearance should be familiar to those skilled in the art. Although inverter harmonic voltage content is greater during square wave operation than during operation in the pulse width modulated mode, the machine load and the machine rotor itself provides sufficient inertia to smooth out any pulsations at the relatively high speed of this operating condition.
One of the difficulties incurred in pulse width modulated inverter machine drive systems is their inability to smoothly transition from pulse width modulation operation to square wave operation. During operation in the pulse width modulation mode, when the sine wave amplitude is increased to increase machine amplitude, it is inevitable that during certain intervals, when the sine wave intercepts the triangle wave, the switching pulse width, that is, the time interval during which the sine wave amplitude exceeds the triangle waveform amplitude, will become smaller than the minimum time required to commutate the then conductive inverter switching device and to render the incoming switching device conductive. To avoid the undesirable effects of too narrow inverter switching pulses, it is desirable to simply drop them. However, if such switching pulses are just dropped, due to the minimum width of these pulses, the inverter output voltage increases sharply, causing appreciable jumps in motor torque. One apparatus for controlling an inverter machine drive system for accomplishing smooth inverter transition from pulse width modulation operation to square wave operation is described in my U.S. Pat. No. 4,047,083 issued on Sept. 6, 1977 and assigned to the General Electric Company. The control apparatus of my previous patent implements pulse width modulation operation during intervals of low machine speed by supplying the inverter with switching pulses produced in accordance with the conventional triangulation technique. At high machine speeds, the control apparatus supplies switching pulses to the inverter which are generated in accordance with a dual direct current level set scheme with the lower level varied in accordance with a higher level, whose magnitude is varied in accordance with an operator commanded amplitude signal. While a control apparatus described and claimed in my previous patent allows the inverter to smoothly transition from pulse width modulation operation to square wave operation its complexity makes it difficult to implement.
In contrast, the present invention concerns a low complexity control apparatus for a pulse width modulated inverter-machine drive system which accomplishes smooth inverter transition from pulse width modulation operation to square wave operation.
It is an object of the present invention to provide a low complexity control apparatus for a pulse width modulated inverter-machine drive system which implements pulse width modulation inverter operation during intervals of low machine frequency, square wave operation during intervals of high machine frequency and which implements smooth transitioning from pulse width modulation to square wave inverter operation.
It is a further object of the present invention to provide a control apparatus for a pulse width modulated inverter-machine drive system which accomplishes smooth transitioning from pulse width modulation to square wave inverter operation without objectionable jumps in machine voltage or high inverter voltage harmonic content.